Recently, secondary batteries have been an essential and important constituent element as power sources of personal computers, video cameras, mobile phones, and the like, or as power sources for automobiles and power storage.
Among secondary batteries, in particular, lithium ion secondary batteries are characterized as having higher energy density than other secondary batteries and operability at high voltage. Accordingly, lithium ion secondary batteries are secondary batteries that can be easily made compact and lightweight and used in information-related equipment and communication equipment. In recent years, lithium ion secondary batteries with high power and high capacity have been under development for use in electric cars and hybrid cars as low-emission vehicles.
Lithium ion secondary batteries or lithium secondary batteries include a positive electrode layer, a negative electrode layer, and a lithium salt-containing electrolyte arranged between the electrodes. The electrolyte is composed of a nonaqueous liquid or a solid. When the electrolyte is a nonaqueous liquid electrolyte, the electrolyte liquid infiltrates into the positive electrode layer, facilitating the formation of the interface between a positive electrode active material constituting the positive electrode layer and the electrolyte, thus easily improving performance. However, it is necessary to dispose a safety device for suppressing temperature increase during a short circuit or mount a system for ensuring safety, such as short circuit prevention, since widely used electrolyte liquids are flammable. In contrast, all-solid batteries, which are batteries entirely composed of solid components by using a solid electrolyte instead of a liquid electrolyte, do not contain any flammable organic solvent. Thus, all-solid batteries are expected to achieve simplification of safety devices and be advantageous in production cost and productivity, and therefore, the development of the batteries is being promoted.
In all-solid batteries in which a solid electrolyte layer is arranged between the positive electrode layer and the negative electrode layer, the electrolyte hardly infiltrates into the positive electrode active material, easily reducing the interface between the positive electrode active material and the electrolyte, since the positive electrode active material and the electrolyte are solid. Therefore, in all-solid batteries, an area of the interface is increased by using, as a positive electrode, a mixture containing a mixed powder obtained by mixing a powdered positive electrode active material and a powdered solid electrolyte.
In particular, as a solid electrolyte for all-solid batteries, a sulfide-based solid electrolyte having excellent lithium ion conductivity has been considered. However, there is a problem of an easily increasing interface resistance during the movement of lithium ions on the interface between the active material and the sulfide-based solid electrolyte. The reason for this may be that the reaction between the active material and the sulfide-based solid electrolyte allows a high resistance region to be formed on the surface of the active material. Since an increase in the interface resistance deteriorates the performance of all-solid battery, some techniques for suppressing an increase in the interface resistance have been disclosed in the past. One example of the disclosed techniques is reduction of interface resistance by coating of the surface of an active material with lithium niobate or the like (Patent Document 1).